|
| Carbon Cloth Substrate PbO2 Anodes |
Graphite cloth is a useful material that can be purchased from most stores that carry fibreglass. It is conductive and recently I've been experimenting with it as a substrate material for lead dioxide electrodes.
The process for plating substrates in general with lead dioxide is well described on both wouter's page and michael's page. Therefor, I will only discuss things that are particular to plating carbon fibre here.
It is important that the cloth strip to be plated is well secured and preferably taught in the plating bath. A loose strip will tend to drift around the solution and will come out really warped. Usually I either glue some mass to the bottom or wrap the cloth tightly around a frame.
The temporary connection to the graphite can be made from just about any metal either above or below the solution line. The picture on the left shows a large unplated anode clamped between two strips of titanium. The idea here was to plate PbO2 over the titanium/carbon junction to produce a sound electrical connection. Unfortunately, this didn't work so well in the chlorate cells and even at 10 amps the junction overheated. On the right is a setup with four 1" wide strips of graphite cloth stretched over a plexiglass frame. The connection is made with strips of brass clamped to the carbon just above the solution level.
I use a nitrate based plating solution which started out similar to the one on Wouter's page, only with considerably more nitric acid (250-300 g/L Pb(NO3)2, 50 g/L CuCO3, 50 g/L HNO3). After each usage additional lead and copper carbonate is added to make up for that lost during the cycle and who knows what's in their now.
The plating current is usally between 0.02 - 0.06 amps/cm². At this rate it takes 12-24 hours to build a sufficient thickness of lead dioxide.
Since very few bubbles are created by the electrolysis, it is a necessary to stir the solution mechanically during plating. An easy way to do with is by bubbling air with an aquarium pump.
If all went well, the electrode will come out of the bath uniformly coated with shiny black lead dioxide. The weave will clearly show through but the coating should not be nodular, except perhaps around the edges. Because of the fibre reinforcement, these anodes are strong and resilient, wheras most lead dioxide electrodes are quite frail.
To use this electrode in a chlorate or perchlorate cell it is necessary to make a reliable electrical connection to the lead dioxide. The use of silver for this task is well established and provides for an almost zero resistance contact. I cut a sliver off a silver block purchased from a jewelery store and pound it flat with a hammer and anvil. This can be wrapped around the anode and then a copper strip is placed over the silver and the whole thing is wrapped tightly with masking tape, as seen on two of the above anodes.
